The invention relates to a temperature detection device for an electric radiant heater, with which is associated an active sensor for the detection, of the positioning of a cooking vessel on a hotplate covering the radiant-heater and in particular a glass ceramic plate.
The automatic switching on and off of a cooking point as a direct function of the placing thereon of a cooking vessel or pot has been a long-term aim. The systems proposed for this purpose are based on the most varied principle and usually the nature and arrangement of the sensor for detecting the cooking vessel positioning is decisive. In the inductive systems considered in preferred manner here, the sensor is part of an inductive resonant circuit of a control means, preferably operating by means of resonant circuit tuning, and has at least one sensor loop with electrically conductive material through which an inductance is formed. The sensor loop is positioned in the vicinity of at least one heating zone heatable by electric radiant heating elements in such a way that through a cooking vessel positioned in the vicinity of the heating zone the inductance of the sensor loop is modified in such a way that a connected evaluating device can distinguish between the presence and absence of a set down cooking vessel.
A simple and robustly constructed pot detection system of this type, which supplies particularly significant signals for the control of the radiant heater is disclosed by DE 196 03 845. A temperature detection device for the radiant heater therein comprises a temperature monitor with a rod-like temperature sensor, which acts on a temperature monitor contact for maintaining a permitted material temperature on the underside of the glass ceramic hotplate and on a hot indication contact for indicating the hot state of the heater. The rod sensor projects through an insulator rim laterally bounding the heating zone and passes in a plane above the radiant elements below the sensor loop.
The problem of the invention is to provide a temperature detection device for such radiant heaters, which is particularly simple and inexpensively manufacturable.
This problem is solved by a temperature detection device having the features of claim 1. Preferred further developments are given in the dependent claims, whose wording is by reference made into part of the content of the description.
According to the invention the sensor loop has at least one portion, which is functionally part of a temperature sensor, which emits temperature signals, of the temperature detection device, the temperature signals preferably being electrical and/or are electrically or electronically evaluatable. The presence of a sensor loop for pot detection purposes is consequently utilized in order to ensure an adequately precise temperature detection with the aid of the sensor loop. Use is made of the fact that the sensor loop is generally located in the vicinity of a heating zone and preferably also in the immediate proximity of the hotplate, particularly glass ceramic plate, whose temperature is to be monitored. This favourable positioning of the sensor loop or its portion used for temperature detection purposes makes it possible, particularly in conjunction with appropriately selected electrical and/or structural characteristics of the portion or the sensor loop, to integrate an effective temperature detection device with the pot detection sensor system. Thus, there is no need for separately provided temperature detection devices, such as e.g. the aforementioned rod sensors.
A further development is characterized in that the sensor loop at least partly overlaps the heating zone with at least one overlapping portion and the portion used as the functional part of the temperature detection device is preferably located in the vicinity of the overlapping portion. As a result the portion used for temperature detection purposes is located directly in the radiant area of the radiant elements and preferably spaced from an insulating edge bounding the heating zone. This permits a particularly short delay temperature detection or temperature change detection with optionally only small temperature variations. This arrangement also leads to advantages for the pot detection function, because a pot detection signal provides much more information for the covering of the heating zone as compared with a sensor passing round in the marginal area of the heater and consequently is more significant for pot detection purposes. The relevant explanations in DE 196 03 845 are hereby, by reference, made into part of the present application.
Within the scope of the invention various appropriate use possibilities of the sensor loop exist for temperature detection. For example the situation can be such that the temperature detection device has an electronic device for evaluating electrical temperature signals and that said device is in signal-conducting, electrical connection with the portion of the sensor loop. The portion can here form an electrically active part of the temperature sensor, so that temperature-dependent, electrical characteristics of the portion, optionally in conjunction with the temperature-dependent, electrical characteristics of neighbouring portions, can be used for producing temperature signals.
The situation can e.g. be such that the device for evaluating temperature signals is constructed for detecting and evaluating thermally caused changes in the electrical resistance of the portion, the complete sensor loop or a resistance element carried by the sensor loop, e.g. a resistance wire or a resistive film. For producing very readily evaluatable temperature signals the material, whose resistance is used for temperature detection purposes, is appropriately provided with a high temperature coefficient of the electrical resistance and said temperature coefficient can be both positive (PTC) and negative (NTC).
It is also possible for the sensor loop with at least one loop portion to form part of a temperature sensor functioning as a thermocouple. A connected signal processing is then appropriately provided for the processing of thermoelectric voltages and can assume any appropriate form for this.
The construction of at least one thermocouple with the aid of the sensor loop or one of its portions can take place in different ways. For example, the sensor loop can have a first loop portion of a first electrically conductive material and a second loop portion, contacted therewith, of a second electrically conductive material, said first and said second materials having different contact potentials in the contact series. The contact point is appropriately located in the vicinity of a preferably provided overlapping portion, i.e. is directly subject to the radiant energy of the radiant heater. An advantage of this variant is its simple construction, because apart from the sensor loop no additional elements are required. The thermoelectric voltage can simply be tapped at appropriate points of the resonant circuit embracing the sensor loop.
For forming a thermocouple it is also possible to provide at least one preferably filamentary material portion, which is made from an electrically conductive material with an electric contact potential different from the loop portion material and which is electrically conductively connected to the loop portion in the vicinity of a contact point and is more particularly welded thereto. In this case one thermocouple side is formed by the loop portion, whereas the separate material portion fitted thereto forms the other side. The thermoelectric voltage is then tappable between the end of the material portion and one end of the sensor loop. For forming a thermocouple with two contact areas, it is also possible to provide two preferably filamentary material portions of electrically conductive material, which are electrically conductively connected in spaced manner with a loop portion in the vicinity of the contact points and are more particularly welded, the two material portions being made from materials with different electric contact potentials. In this case a thermoelectric voltage can be tapped at the free ends of both material portions and said voltage is essentially only determined by the different contact potentials of these materials. There is here a substantially complete freedom in the choice of material for the loop portion.
The latter variants with separate material portions fitted and in particular welded to the sensor loop with the formation of contact points offer the advantage that the sensor loop can have a very simple construction. By fitting suitable, e.g. filamentary material portions, e.g. by spot welding, it is also possible to reequip in simple inexpensive manner for the formation of temperature detection devices according to the invention existing systems with pot detection sensors.
It is also possible for the device for detecting and evaluating temperature signals to be constructed so as to emit at least one measurement pulse and to have a device for determining the transit time of said pulse through the sensor loop or through a separate measuring element, more particularly carried by and associated with the sensor loop. Use is made of the fact here that the transit time of a measurement pulse, e.g. through the sensor loop, is generally extended by the heating of the sensor. This transit time change can be used as a measure for the temperature. It is also possible to use the presence of the pot detection sensor in such a way that a portion located in the vicinity of the heating zone uses the same as a heat absorption portion, dissipates the absorbed heat along the sensor loop and at another point, e.g. in the vicinity of an insulating edge or outside the heating zone utilizes the same for temperature detection purposes. In this case the temperature sensor can e.g. have at least one temperature switch in thermally conductive connection with the portion, particularly the overlapping portion and which is fitted to the sensor loop for the temperature-dependent short-circuiting of at least one turn of the sensor loop. The temperature switch can e.g. be constructed as a snap action switch in the manner of a bimetallic switch. The temperature-dependent switching of the temperature switch and the associated short-circuit or elimination of a short-circuit of sensor loop turns leads to a temperature-dependent inductance jump, which can be readily evaluated with the evaluation electronics of the pot detection system. Such an induction jump dependent on the switching temperature of the temperature switch can e.g. be used in the framework of a temperature monitor (overheating protection) and/or in the framework of a hot indication.
It is also possible that the sensor loop or a suitable portion thereof is constructed as a support for at least one electrically active element of a temperature sensor. The support function more particularly means that the portion maintains the electrically active element of the temperature sensor in a position favourable for temperature detection, e.g. spaced from the insulating edge of the heating zone above the radiant heating elements. The sensor loop can exclusively have this support function or can additionally be used for some other, e.g. electrical function.
Thus, it is in particular possible for the sensor loop to have a preferably tubular hollow body made from thermally stable, electrically conductive material and for there to be in an inner area of the hollow body at least one electrically active, preferably filamentary inner element of the temperature sensor. The hollow body can serve not only as a support for the inner element, but also at the same time as a protection thereof against mechanical damage and/or thermally caused deterioration to characteristics. For example, the inner element can be an element of a thermocouple, e.g. a leg thereof, or also a complete thermocouple (with two elements connected to a contact point). The wire material for forming a thermocouple can in the case of a supporting and protecting envelope be much thinner and therefore less expensive than in the case of an optionally self-supporting and/or exposed thermocouple. The preferably inherently rigid or self-supporting hollow body, e.g. an iron-nickel-chrome alloy tube can itself be an electrically active part of a thermocouple, in that the other partial element is appropriately contacted with the hollow body e.g. by spot welding. The inner element can also be made from an electrically conductive material portion with a high positive or negative temperature coefficient of the electrical resistance in order to utilize the aforementioned resistance measurement for temperature detection purposes. Except for the area of optionally desired contact points, the inner element and hollow body are appropriately insulated against one another, in that e.g. the inner element is surrounded by a ceramic insulating material, which partly or entirely fills the interior of the hollow body.
If the temperature detection device according to the invention is used for continuous temperature detection purposes, it is e.g. possible to replace conventional rod regulators or the like. It is also possible to control a hot indicating device as a function of the real hotplate, particularly glass ceramic plate temperature and optionally to indicate the same. In general numerous functions can be implemented where the detection of a current hotplate temperature is important. For example, no longer need fixed settings of a setting regulator be associated with cooking stages of a cooking implement and instead, as in so-called automatic hotplates, they can be associated with the current temperature detected by the temperature detection device and to which they are set. A temperature regulating means can also be designed in such a way as to automatically control the function of a parboiling surge.
Any suitable sensor loop of a pot detection device can be utilized for the construction of temperature detection devices according to the invention. A preferred embodiment in which the sensor loop only has a single turn of dimensionally stable, self-supporting and electrically conductive material will be described in greater detail in conjunction with the embodiments. It can be in the form of a solid, thick wire or in the form of a tube, whose interior can be used for receiving elements of the temperature detection device. As a result of an advantageous arrangement of the sensor loop directly below the hot point with a significant spacing from the radiant elements it can be ensured that the prevailing temperature on the sensor loop corresponds with respect to its path and absolute amount to that of the hotplate.
The invention also relates to the use of at least one portion of a sensor loop of an inductive sensor for detecting the positioning of a cooking vessel on a hotplate, particularly glass ceramic plate covering a radiant heater, as the functional part of a temperature sensor for determining the temperature of the hotplate. As stated, the portion or the entire sensor loop can be electrically active or, alternatively or additionally, can serve as a support for at least one electrically active element of a temperature sensor. The particular advantage is that the sensor loop or portion can be positioned particularly favourably for a temperature detection, more particularly close to the underside of a glass ceramic plate and/or that if appropriate it is possible to obviate the need for separate elements for creating a temperature sensor, because the sensor loop fulfils a double function both within the framework of an electrical temperature sensor and within that of an inductive sensor for pot detection.
Apart from the claims, these and further features can be gathered from the description and drawings and the individual features, both singly or in the form of subcombinations, can be implemented in an embodiment of the invention and in other fields and can represent advantageous, independently protectable constructions.